1. Field of the Invention
The invention relates to an interconnection (contact) element suitable for effective pressure connections between electronic components.
2. Description of Related Art
Interconnection or contact elements may be used to connect devices of an electronic component or one electronic component to another electronic component. For example, a contact element may be used to connect two circuits of an integrated circuit chip or including an application specific integrated circuit (ASIC). Contact elements may also be used to connect the integrated circuit chip to a chip package suitable for mounting on a printed circuit board of a computer or other electronic device. Contact elements may further be used to connect the integrated circuit chip to a test device such as a probe card assembly or other printed circuit board (PCB) to test the chip.
Generally, interconnection or contact elements between electronic components can be classified into at least the two broad categories of xe2x80x9crelatively permanentxe2x80x9d and xe2x80x9creadily demountable.xe2x80x9d
An example of a xe2x80x9crelatively permanentxe2x80x9d contact element is a wire bond. Once two electronic components are connected to one another by a bonding of a contact element to each electronic component, a process of unbonding must be used to separate the components. A wire bond contact element, such as between an integrated circuit chip or die and inner leads of a chip or package (or inner ends of lead frame fingers) typically utilizes a xe2x80x9crelatively permanentxe2x80x9d contact element.
One example of a xe2x80x9creadily demountablexe2x80x9d contact element is the contact element between rigid pins of one electronic component received by resilient socket elements of another electronic component. A second type of a xe2x80x9creadily demountablexe2x80x9d contact element is a contact element that itself is resilient or spring-like or mounted in or on a spring or resilient medium. An example of a contact element is a tungsten needle of a probe card component. The contact element of a probe card component is typically intended to effect a temporary pressure connection between an electronic component to which the spring contact element is mounted and terminals of a second electronic component, such as a semiconductor device under test.
With regard to spring contact elements, generally, a certain minimum contact force is desired to effect reliable pressure contact to an electronic component (e.g., to terminals on electronic component). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per terminal) may be desired to effect a reliable electrical pressure connection to a terminal of an electronic component.
A second factor of interest with regard to spring contact elements is the shape and metallurgy of the portion of the spring contact element making pressure connection to the terminal of the electronic component. With respect to the tungsten needle as a spring contact element, for example, the contact end is limited by the metallurgy of the interconnection element (e.g., tungsten) and, as the tungsten needle becomes smaller in diameter, it becomes commensurately more difficult to control or establish a desired shape at the contact end.
In certain instances, spring contact elements themselves are not resilient, but rather are supported by a resilient membrane. Membrane probes exemplify this situation, where a plurality of microbumps are disposed on a resilient membrane. Again, the technology required to manufacture such contact elements limits the design choices for the shape and metallurgy of the contact portion of the contact elements.
Commonly-owned U.S. patent application Ser. No. 08/152,812 filed Nov. 16, 1993 (not U.S. Pat. No. 4,576,211, issued Dec. 19, 1995), and its counterpart commonly-owned co-pending xe2x80x9cdivisionalxe2x80x9d U.S. patent application Ser. No. 08/457,479 filed Jun. 1, 1995 (status: pending) and Ser. No. 08/570,230 filed Dec. 11, 1995 (status: pending), all by Khandros, disclose methods for making spring contact elements. In a preferred embodiment, these spring contact elements, which are particularly useful for micro-electronic applications, involve mounting an end of a flexible elongate core element (e.g., wire xe2x80x9cstemxe2x80x9d or xe2x80x9cskeletonxe2x80x9d) to a terminal on an electronic component, coating the flexible core element and adjacent surface of the terminal with a xe2x80x9cshellxe2x80x9d of one or more materials. One of skill in the art can select a combination of thickness, yield strength, and elastic modulus of the core and shell materials to provide satisfactory force-to-deflection characteristics of the resulting spring contact elements. Exemplary materials for the core element include gold. Exemplary materials for the coating include nickel and its alloys. The resulting spring contact element is suitably used to effect pressure, or demountable, interconnections between two or more electronic components, including semiconductor devices.
Commonly-owned, co-pending U.S. patent application Ser. No. 08/340,144 filed Nov. 15, 1994 and its corresponding PCT patent application No. PCT/US94/13373 filed Nov. 16, 1994 (WO95/14314, published May 16, 1995), both by Khandros and Mathieu, disclose a number of applications for the aforementioned spring contact elements, and also disclose techniques for fabricating contact pads at the ends of the spring contact elements. For example, a plurality of negative projections or holes, which may be in the form of inverted pyramids ending in apexes, are formed in the surface of a sacrificial layer (substrate). These holes are then filled with a contact structure comprising layers of material such as gold or rhodium and nickel. A flexible elongate element is mounted to the resulting contact element structure and can be overcoated in the manner described hereinabove. In a final step, the sacrificial substrate is removed. The resulting spring contact element has a contact pad having controlled geometry (e.g., a sharp point) at its free end.
Commonly-owned, co-pending U.S. patent application Ser. No. 08/452,255 filed May 26, 1995 and its corresponding PCT Patent Application No. PCT/US95/14909 filed Nov. 13, 1995 (WO96/17278, published Jun. 6, 1996), both by Eldridge, Grube, Khandros and Mathieu, disclose additional techniques and metallurgies for fabricating contact tip structures on sacrificial substrates, as well as techniques for transferring a plurality of spring contact elements mounted thereto, en masse, to terminals of an electronic component.
Commonly-owned, co-pending U.S. Provisional Patent Application No. 60/005,189 filed May 17, 1996 and its corresponding PCT Patent Application No. PCT/US96/08107 filed May 24, 1996 (WO96/37332, published Nov. 28, 1996), both by Eldridge, Khandros and Mathieu, disclose techniques whereby a plurality of contact tip structures are joined to a corresponding plurality of elongate contact elements that are already mounted to an electronic component. Also disclosed are techniques for fabricating xe2x80x9celongatexe2x80x9d contact tip structures in the form of cantilevers. The cantilever tip structures can be tapered, between one end thereof and an opposite end thereof. The cantilever tip structures are suitable for mounting to already-existing (i.e., previously fabricated) raised contact elements extending (e.g., free-standing) from corresponding terminals of an electronic component.
Commonly-owned, co-pending U.S. Patent Application No. 60/024,555 filed Aug. 26, 1996, by Eldridge, Khandros and Mathieu, representatively discloses a technique whereby a plurality of elongate tip structures having different lengths than one another can be arranged so that their outer ends are disposed at a greater pitch than their inner ends. The inner, xe2x80x9ccontactxe2x80x9d ends may be collinear with one another, for effecting connections to electronic components having terminals disposed along a line, such as a center line of the component.
As electronic components get increasingly smaller and the spacing between terminals on the electronic components get increasingly tighter or the pitch gets increasingly finer, it becomes increasingly more difficult to fabricate interconnections including spring contact elements suitable for making electrical connection to terminals of an electronic component. Co-pending and commonly-owned U.S. patent application Ser. No. 08/802,054, titled xe2x80x9cMicroelectronic Contact Structure, and Method of Making Same,xe2x80x9d discloses a method of making spring contact elements through lithographic techniques. In one embodiment, that application discloses forming a spring contact element (including a spring contact element that is a cantilever beam) on a sacrificial substrate and then transferring and mounting the contact element to a terminal on an electronic component. In that disclosure, the spring contact element is formed in the substrate itself through etching techniques. In co-pending, commonly-owned U.S. patent application, Ser. No. 08/852,152, titled xe2x80x9cMicroelectronic Spring Contact Elements,xe2x80x9d spring contact elements are formed on a substrate, including a substrate that is an electronic component, by depositing and patterning a plurality of masking layers to form an opening corresponding to a shape embodied for the spring contact element, depositing conductive material in the opening made by the patterned masking layers, and removing the masking layer to form the free-standing spring contact element.
Co-pending and commonly-owned U.S. patent application, Ser. No. 09/023,859, titled xe2x80x9cMicroelectronic Contact Structures and Methods of Making Same,xe2x80x9d describes a contact element having a base end portion (post component), a middle portion (beam component) and a contact end portion (tip component) and methods separately forming each portion and joining the post portion together as desired on an electronic component.
What is needed is a method of fabricating interconnections suitable for present fine-pitch electrical connections that is scalable for future technologies. Also needed are improved methods of making contact elements particularly methods that are repeatable, consistent, and inexpensive.
A method of forming an interconnection, including a spring contact element, by lithographic techniques. In one embodiment, the method includes applying a masking material over a first portion of a substrate, the masking material having an opening which will define a first portion of a spring structure, depositing a structure material (e.g., conductive material) in the opening, and overfilling the opening with the structure material, removing a portion of the structure material, and removing a first portion of the masking material. In this embodiment, at least a portion of the first portion of the spring structure is freed of masking material. In one aspect of the invention, the method includes planarizing the masking material layer and structure material to remove a portion of the structure material. In another aspect, the spring structure formed includes one of a post portion, a beam portion, and a tip structure portion.
The techniques presented herein may be used to form, for example, contact elements including spring contact elements on substrates including electronic components, such as ceramic- or semiconductor-based components. The contact elements may be formed, in whole or in part, directly on an electronic component or formed separately, as on a sacrificial substrate, and transferred to an electronic component.
The method presented herein provides an improved method for fabricating a contact element. The use of lithographic techniques to fabricate the contact element is well-suited to the fine-pitch, close-tolerance world of micro-electronic components. The invention addresses and is particularly well-suited to making interconnections to electronic components having their terminals (e.g., bond pads) disposed at a fine-pitch. As used herein, the term xe2x80x9cfine-pitchxe2x80x9d refers to electronic components that have their terminals disposed at a spacing of at least less than 5 mils, such as 2.5 mils or 65 xcexcm. As will be evident from the description that follows, this is preferably achieved by taking advantage of the close tolerances that readily can be realized by using lithographic rather than mechanical techniques to fabricate the contact elements. The incorporation of planarization steps into embodiments of the method of the invention permits multiple contact elements to be fabricated on a substrate with minimal tolerances. Thus, for example, a plurality of spring contact elements having a structure such as described above may be fabricated on or transferred to an electronic component such that each of their tip portions have a similar height and when contacted, for example, by a second electronic component, are displaced a similar distance. In this manner, the contact elements formed by the method of the invention offer more reliable contact height and contact force than prior art structures.
A contact element is also disclosed. In one aspect of the invention, the contact element is a resilient contact element or spring contact element in the form of a cantilever that includes an elongate beam portion extending about the first axis and having a surface that is substantially parallel to a second surface, a post portion coupled to a first surface and extending in a first direction by a second axis, and a tip portion coupled to a second surface and extending in a second direction above the second axis. The post portion is adapted to be coupled to an electronic component, such as to terminals on a ceramic- or a semiconductor-based electronic component. The beam portion forms a cantilever supported at one end to the post portion with the tip portion coupled to a second end.
The contact element of the invention is suitable for making either temporary or permanent electrical connection between terminals of electronic components such as a PCB and a chip under test. The contact element may be fabricated as a permanent element directly on an electronic component, such as a space transformer of a probe card assembly. Alternatively, the contact element of the invention may be separately fabricated on a sacrificial substrate and affixed at its post end, for example, by soldering to on electronic component.
For making temporary connection, the electronic component upon which the contact element is fabricated is brought together with another electronic component so that the tip end of the spring contact element is in pressure contact with a terminal of the other electronic component. The contact element reacts resiliently to maintain contact pressure and any electrical connection between the two components.
For making permanent connections, the electronic component upon which the contact element is fabricated or affixed is brought together with another electronic component, and the tip end of the contact element is joined or bonded, such as by soldering or brazing or with a conductive adhesive, to a terminal of the other electronic component. In one embodiment, the contact element is compliant and may accommodate differential thermal expansion between the two electronic components.
As noted above, the contact element of the invention can be fabricated, along with a plurality of other contact elements, directly on the surface of a device, such as a semiconductor device, or on the surfaces of a plurality of semiconductor devices resident on a semiconductor wafer. In this manner, a plurality of devices resident on a semiconductor wafer can be xe2x80x9creadiedxe2x80x9d for burn-in and/or test prior to being singulated from the semiconductor wafer. Alternatively, the contact element of the invention can be fabricated, typically with a plurality of other contact elements, on a sacrificial substrate and transferred to an electronic component.